Hybrid models of magnetized discharge plasmas: fluid electrons + particle ions

1. Hybrid models of magnetized discharge plasmas: fluid electrons + particle ions Gerjan Hagelaar Centre de Physique des Plasmas et de leurs Applications de Toulouse Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 9, France

2. Introduction Magnetic fields used in low-pressure discharges: • magnetron • electron-cyclotron resonance (ECR) • helicon • Hall-effect thruster • etc… (magnetized discharges) Magnetic field  complex physics Insight from hybrid models

3. Plan • Elementary physics • Hybrid models • Limits of hybrid models • Illustrative model results: - ECR reactor - Hall thruster - Galathea trap

4. Elementary effects of the magnetic field • Cyclotron motion  confinement • Perpendicular electric field  EB drift • Collisions destroy magnetic confinement ion electron electron EB drift (azimuthal) cyclotron frequency Larmor radius collision E B B

5. Typical conditions plasma pressure 0.1 – 10 mTorr plasma density 1015 – 1019 m-3 magnetic field 0.001 – 0.1 T electron temperature 2 – 20 eV lengths Debye length 10-5 – 10-3 m electron Larmor radius 10-4 – 0.01 m ion Larmor radius 0.02 – 5 m mean free path 0.01 – 1 m plasma size 0.02 – 1 m frequencies electron cyclotron 3108 – 21010 s-1 electron collision 3105 –108 s-1 Long mean free path Electrons are magnetized  collisions + ionization Ions have only few collisions Magnetic field not influenced by plasma

6. Modelling Low pressure  particle-in-cell (PIC): • electron and ion trajectories • space charge electric fields Magnetized PIC models cumbersome: • high plasma density  small time steps, small cells • important 2D effects • interest in simpler faster models • describe electrons by collisional fluid equations K. A. Ashtiani et al, J. Appl. Phys. 78 (4), 2270-2278 (1995). S. Kondo and K. Nanbu, J. Phys. D: Appl. Phys. 32, 1142-1152 (1999). J. C. Adam et al, Phys. Plasmas 11 (1), 295-305 (2004).

7. Electron fluid equations • Electron conservation • Anisotropic flux • Mobility tensor (classical theory) ionisation source flux drift diffusion collision frequency cyclotron frequency perpendicular mobility << parallel mobility

8. Hybrid models Non-quasineutral scheme: • ion particles  ni • electron fluid  ne • Poisson   Quasineutral scheme: • ion particles  ni = ne • electron fluid   no plasma oscillations • large time steps no sheaths  large cells (Ohm’s law) R. K. Porteous et al, Plasma Sources Sci. Technol. 3, 25-39 (1994). J. M. Fife, Ph. D. thesis, MIT, 1998. G. J. M. Hagelaaret al, J. Appl. Phys. 91 (9), 5592-5598 (2002).

9. Limits of the electron equations • Anomalous transport B empirical parameters • Non-local effects //B:inertia, mirror confinement But: flux //B limited by boundaries classical mobility ? Bohm mobility drift diffusion  (Boltzmann) potential = constant + diffusion term Magnetic field lines approximately equipotential

10. insulator wall  h  uniform B cathodec anode a   l  insulator wall Numerical problem Extreme anisotropy  numerical errors tend to destroy the magnetic confinement Special precautions necessary (flux scheme) electron flux in the middle of the channel [cyclotron frequency] / [collision frequency]

11. Examples of model results Non-quasineutral hybrid model  sheaths resolved Fixed: • Gaussian ionisation source • uniform electron temperature (diffusion) • electron collision frequency Calculated: • electron/ion densities • electron/ion fluxes, currents • self-consistent potential

12. Example I : Diffusion in ECR reactor process chamber source chamber

13. ECR reactor with dielectric wall no (pre)sheath !! Magnetic confinement reduces loss to source wall

14. ECR reactor with grounded wall normal (pre)sheath current loop Magnetic confinement shortcircuited by walls A. Simon, Phys. Rev. 98 (2), 317-318 (1955).

15. Example II : Hall-effect thruster

16. Hall-effect thruster cathode sheath negligible ion beam trapped low-energy ions acceleration region Equipotential lines  magnetic field lines Applied voltage penetrates in plasma bulk

17. Example III : semi-Galathea trap A. I. Morozov and V. V. Savel’ev, Physics – Uspekhi 41 (11), 1049-1089 (1998).

18. Semi-Galathea trap 70 % of ions guided to exit negative plasma potential ! (inverted presheath) electron current from emissive cathode to walls Potential well reduces ion wall loss and guides ions to exit

19. Semi-Galathea trap without emission cathode sheath Potential well disappears because of cathode sheath

20. Conclusions • In magnetized discharges, charged particle transport and space charge fields are different • This can be studied in 2D by hybrid models • No predictive simulations, but insight in physical principles